The connection between oceans and climate change is clear and undisputed. Oceans control the climate system by absorbing heat and carbon dioxide, while climate change from greenhouse gas (GHG) emissions causes ocean acidification, ocean warming and sea level rise, threatening many low-lying island nations. These island nations are urgently seeking protection under international law.

A crucial step in this effort was achieved on 21 May 2024. For the first time, an international court addressed the obligations of the 168 States parties to the UN Convention on the Law of the Sea (UNCLOS), including Australia, concerning climate change impacts on the oceans, such as ocean acidification, and sea level rise. The International Tribunal for the Law of the Sea (ITLOS) ruled unanimously that anthropogenic GHG emissions absorbed by the oceans constitute 'pollution of the marine environment' due to their harmful effects on marine ecosystems and marine biodiversity. This means GHG emissions, introduced indirectly into the oceans, whether from CO2-emitting industrial facilities, airplanes, or vessels, whether land-based or ocean-based, are now legally recognised as a form of ocean pollution that must be mitigated, controlled, and eventually prevented.

The significance of this finding should not be underestimated, as it triggers States’ international law obligation to take ‘all necessary measures’, individually and collectively, against GHG emissions. In adopting these necessary measures, States should follow the science, notably the scientific conclusions of the Intergovernmental Panel on Climate Change (IPCC). This includes assessing the cumulative carbon footprint of all planned projects on the oceans before allowing them to go ahead.

It also includes an obligation to take all necessary actions to address the impacts of climate change on developing and climate-vulnerable countries, including low-lying island nations, by providing them with economic and other assistance.

While the ITLOS ruling is not legally binding, it holds immense authoritative value in interpreting and applying States’ environmental obligations under UNCLOS to modern climate change challenges. It directs States and courts, both national and international, to apply and enforce these obligations consistently to combat climate change.

It is likely that other international and national courts will consider the ITLOS advisory opinion when making their own assessments on climate-related cases, including challenging the legality of high-emitting activities due to their potential carbon footprint on the oceans.

ITLOS’ ruling is expected to influence climate proceedings at the International Court of Justice and the Inter-American Court of Human Rights, as well as over 2,000 domestic climate litigation cases seeking to hold governments and corporations accountable for inadequate climate action.

Small island nations are seeking protection for the world’s oceans from catastrophic climate change that threatens their existence. The 1982 United Nations Convention on the Law of the Sea (UNCLOS) defines pollution as the introduction by humans of 'substances or energy into the marine environment' that harms marine life, but it does not explicitly define greenhouse gas emissions as a specific pollutant. Neither does it specify minimum standards to reduce greenhouse gas emissions. Climate change is not mentioned in UNCLOS. In fact, many ocean climate change impacts were unthinkable during the negotiations that led to UNCLOS. 

A landmark international decision is expected on 21 May 2024, when the International Tribunal for the Law of the Sea (ITLOS) will address the obligations of the 168 States parties to UNCLOS, including Australia, concerning climate change impacts on the oceans, such as climate-induced ocean warming, sea level rise, and ocean acidification. ITLOS will determine if greenhouse gas emissions absorbed by the oceans can be considered pollution and, if so, whether this requires States to take explicit action against greenhouse gas emissions under UNCLOS.

If ITLOS clarifies these legal obligations, it would make an important contribution to international jurisprudence on global environmental protection and climate change. While an ITLOS Advisory Opinion is not legally binding, it would hold immense authoritative value in interpreting and applying States’ environmental obligations under UNCLOS. It would reinforce the global environment and climate change regime, and it is likely that other international and national courts would consider the ITLOS Advisory Opinion when making their own assessments on climate-related cases, including challenging the legality of high-emitting activities due to their potential carbon footprint on the oceans. ITLOS’ legal determination on the impacts of climate change on oceans and legal obligations to address these, whether through increased environmental due diligence, more robust greenhouse emission reduction policies or environmental impact regulations, will likely be relied upon in future climate cases.

The law of the sea regime established by UNCLOS could, therefore, serve to evaluate States’ actions and inactions, both domestically and internationally, against UNCLOS’ environmental obligations and potentially encourage further efforts to address the impacts of climate change, in line with the general obligation to protect the marine environment from all sources of marine pollution.